Technical Field
The present invention relates to a method of preparing silver nanoparticles. Specifically, the present invention relates to a method involving sputtering and annealing to form silver nanoparticles on a zinc oxide thin film surface wherein the average particle size, particle size distribution, density and shape can be controlled and manipulated. The silver nanoparticles can be used in applications including gas sensing, solar cells and photocatalysis.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Metal nanoparticles exhibit unprecedented and exciting properties strongly dependent on their size, shape and inherent electrons distribution with reference to those of macro-scaled counterpart (G. L. Hornyak, H. F. Tibbals, J. Dutta and J. J. Moore, Introduction to Nanoscience and Nanotechnology, CRC Press of Taylor and Francis Group LLC, ISBN-13: 9781420047790 (2008)—incorporated herein by reference in its entirety). Thus, their applications in medicine, sensors, cosmetics, renewable energies, oncology, etc. are revolutionary in our modern science and technology (G. L. Hornyak, H. F. Tibbals, J. Dutta and J. J. Moore, Introduction to Nanoscience and Nanotechnology, CRC Press of Taylor and Francis Group LLC, ISBN-13: 9781420047790 (2008); J. Dutta and H. Hofmann, Self-Organization of Colloidal Nanoparticles, Encyclopedia of Nanoscience & Nanotechnology, 9, 617 (2004); M. De, P. S. Ghosh and V. M. Rotello, Adv. Mater., 20, 4225 (2008); A-H. Lu, E. L. Salabas and S. Ferdi, Angew. Chem. Int. Ed. Engl., 46, 1222 (2007); C. R Ghosh and Paria S, Chem. Rev., 112, 2373 (2012)—each incorporated herein by reference in its entirety). Amongst the different types of metal nanoparticles, silver nanoparticles (Ag-NPs) are known for their unique physical, chemical and biological properties. Enhanced electrical and thermal conductivity, surface-enhanced Raman scattering, chemical stability, catalytic activity, enhanced photocurrent induction, etc. are some of distinguished physico-chemical properties of Ag-NPs responsible for their potential value in microelectronics, sensing, optoelectronics, corrosion and biomedical (A. H. Alshehri, M. Jakubowska, A. Mlozniak, M. Horaczek, D. Rudka, C. Free and J. D. Carey, Appl. Mater. Interfaces, 4, 7007 (2012); C-W. Chou, S-H. Hsu, H. Chang, S-M. Tseng and H-R. Lin, Poly. Deg. and Stab. 91,1017 (2006); M. K. Hossain, Mat. Sci. Forum, 754, 143 (2013); M. K. Hossain, G. R. Willmott, P. G. Etchegoin, R. J. Blaikie and J. L. Tallon, NanoScale, 5, 8945 (2013); M Ahamed, M S Alsalhi and M K Siddiqui, Clin. Chim. Acta, 411, 1841 (2010)—each incorporated herein by reference in its entirety). Apart from these, Ag-NPs are can also be used in a wide range of consumer products such as plastics, soaps, pastes, food, textiles, etc. due to their excellent anti-bacterial and anti-fungal activity (J. Garcia-Barrasa, J. M. Lopez-de-luzuriaga and M. Monge., Cent. Eur. J. Chem., 9, 17 (2011); J. Fabrega, S. N. Luoma, C. R. Tyler, T. S. Galloway and J. R. Lead, Environ. Intert., 37, 517 (2011); P. Dallas, V. K. Sharma and R. Zboril, Adv. Colloid Interface Sci., 166, 119 (2011); J. Turkevich, P. C. Stevenson and J. Hillier, Discuss. Faraday. Soc., 11, 55 (1951)—each incorporated herein by reference in its entirety).
Most of the Ag-NP synthesis methods that have been reported so far employ chemical, physical and photochemical routes. Every route carries some pros and cons with common issues such as costs, uniformity, scalability, purity, etc.
Generally, three main components are involved in the wet chemical synthesis method: (i) metal salts, (ii) reducing agents and (iii) surfactants or stabilizing agents (J. Kimling, M. Maier, B. Okenve, V. Kotaidis, H. Ballot and A. Plech, J. Phys. Chem. B, 110, 15700 (2006); M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman, Chem. Commun., 7, 801 (1994); A. Manna, P. Chen, H. Akiyama, T. Wei, K. Tamada and W. Knoll, Chem. Mater., 15, 20 (2003); S. D. Perrault and W. C. W. Chan, J. Am. Chem. Soc., 131, 17042 (2009); A. J. Christy and M. Umadevi, Adv. Nat. Sci.: Nanosci. Nanotechnol. 3, 035013 (2012); S. K. Ghosh, S. Kundu, M. Mandal, S. Nath and T. Pal, J. Nanopart. Res., 5, 577 (2003)—each incorporated herein by reference in its entirety). The formation of colloids from the reduction of silver salts involves two stages, viz. nucleation and subsequent nanoparticles growth thereof. Therefore, purity, cost and other issues remain the same.
The photo-induced synthetic strategies generally consist of two approaches; (i) photo-physical (e.g. laser ablation) and (ii) photochemical (e.g. microwave-assisted). In laser ablation, the NPs are synthesized through break-down of bulk metals into nanoscale particles, whereas photochemical route gets NPs into ionic precursors. In brief, the NPs are formed by direct photoreduction of a metal source or reduction of metal ions using photo-chemically generated intermediates (J. Siegel, O. Kvitek, P. Ulbrich, Z. Kolska, P. Slepicka and V. Svorcik, Mater. Lett., 89, 47 (2012); Z. A. Lewicka, Y. Li, A. Bohloul, W. W. Yu, V. L. Colvin, Nanotechnology, 24, 115303 (2013)—each incorporated herein by reference in its entirety). In such techniques, purity, scalability and uniformity pose great challenges.
For a physical approach, the metallic NPs can be generally synthesized by evaporation-condensation, thermal-decomposition, arc discharge, etc. Narrow size distribution and almost zero contamination probability are the two key advantages in this approach with reference to other routes. The physical synthesis process of Ag-NPs usually utilizes external energy (in form of thermal, ac power, arc discharge) to produce Ag-NPs with nearly narrow size distribution (H. Liu, X. Zhang, and T. Zhai, Opt Express, 21, 15314 (2013)—incorporated herein by reference in its entirety). The physical approach can produce large quantities of Ag-NPs samples in a single process. This method is also useful in producing Ag-NPs with different sizes and shapes. However, primary costs for investment of equipment are very high.
In addition, fabrication of metal nanorings has been reported by various methods such as lithography, molecular-beam epitaxy, template-assisted deposition, etc. (S. L. Teo, V. K. Lin, R. Marty, N. Large, E. A.n Llado, A. Arbouet, C. Girard, J. Aizpurua, S. Tripathy, and A. Mlayah, Opt Express 18, 22271 (2010); K. L. Hobbs, P. R. Larson, G. D. Lian, J. C. Keay, and M. B. Johnson, Nano Lett., 4, 167 (2004); C. Eminian, F. J. Haug, O. Cubero, X. Niquille and C. Ballif, Prog. Photovolt: Res. Appl., 19, 260 (2011); H. A. Atwater and A. Polman, Nature Mat., 9, 205 (2010)—each incorporated herein by reference in its entirety).
Template-assisted technique and complicated methods and/or sophisticated facilities are usually required to fabricate such metal nanorings. In such scenarios, particles are surrounded by surfactants and thus applications such as sensing and catalytic cannot be realized because of unexpected analytes around these nanoparticles. Pillai et al. showed that embedding silver nanoparticles in the ZnO layer at the back contact of amorphous silicon solar cells leads to a 20% increase of the photocurrent (S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, J. Appl. Phys., 101, 093105 (2007)—incorporated herein by reference in its entirety). Such improvement of device performance by metallic nanoparticles has been attributed to the induction of localized surface plasmons and light scattering that lead to enhanced light trapping (E. Thouti, N. Chander, V. Dutta and V. K. Komarala, J. Opt., 15, 035005 (2013); S. Dengler, C. Kübel, A. Schwenke, G. Ritt, B. Eberle, J. Opt. 14, 075203 (2012)—each incorporated herein by reference in its entirety). Various studies have shown the importance of nanoparticle size control, the morphology and the average distance between nanoparticles area in order to enhance their effect on light scattering (K. Sivaramakrishnan, A. T. Ngo, S. Iyer, and T. L. Alford J. Appl. Phys. 105, (2009)—incorporated herein by reference in its entirety).
In view of the foregoing, a non-limiting objective of the present invention is to provide simple methods of synthesizing Ag-NPs including Ag nanorings of high purity with uniform distribution or dispersion, wherein the size and the density of the NPs (i.e. number of NPs per unit area) can be controlled.